Aging Auditory System: Presbycusis & its Neural Bases. The process of aging in modern, technologically advanced societies is accompanied by a progressive hearing impairment known as presbycusis. This age-related disorder often causes significant communication problems in persons who have had no previous hearing or speech deficits, resulting in impaired relationships at home and in the workplace, as well as increases costs to the individual, their family and society. Key aspects and neural etiologies of age-related hearing loss are poorly understood despite the fact that 30% of persons age 65-74 and 50% of persons 75 and older suffer from significant hearing impairment; stated another way, after reaching age 60 years, the percentage of the population experiencing difficulty in perceiving speech doubles per decade; 16% at 60, 32% at 70, 64% at 80, and virtually everyone beyond the mid-eighty years. The major theme of this proposal is to increase our knowledge of auditory processing deficits of presbycusis and advance our understanding of the neural bases of this aging disorder. Special attention is paid to distinguishing the effects of peripheral hearing loss and central aging effects in presbycusis. This proposal puts forth a five-year series of interrelated hypotheses tested with tightly coupled and truly interdisciplinary experimental methodologies. Disciplines of psychoacoustics, speech perception, audiology, evoked-potential neurophysiology, and reflex psychology are brought to bear on testing hypotheses in human subject populations to further characterize presbycusis. Scientific disciplines of reflex psychology, gross-potential and single-unit neurophysiology, and neuroanatomy at light and electron microscopic levels test hypotheses utilizing animal populations. These investigations in mammals examine the neural bases of hearing loss resulting from aging effects i the peripheral and central auditory systems. In both human and animal populations, comparisons will be made between young and old subject groups to determine aging effect in hearing loss, and comparisons will be made between subjects with cochlear losses and those with normal hearing to assess the effects of peripheral hearing loss in presbycusis. Special attention will be paid to interdisciplinary, repeated-measures experimental designs, quantitative multivariate data analyses, and determining the effects of different types of background noise on neural and perceptual processing. It is hoped that testing hypotheses put forth in the present proposal concerning the neural bases of age related hearing loss will eventually lead to medical interventions that prevent, alleviate or minimize the communication problems of presbycusis.